This invention relates to a brake control system of vehicles.
A vacuum booster has been used ordinarily as a brake booster. The vacuum booster uses an intake negative pressure of an internal combustion engine. A vacuum pump may be used in place of the intake negative pressure of the internal combustion engine. In the latter case, however, the likelihood of a failure of the brake booster is greater than for a conventional system. Accordingly, it is known to suppress any drop of braking force by using a pump of an anti-lock control system when the brake booster is out of order(refer to JP-A-2001-513041 corresponding to International Publication No. WO98/35867).
According to this patent document, however, the method of suppressing the drop of the braking force by the anti-lock control system at the time of the trouble of the brake booster is not clear and concrete. When the drop of the braking force is suppressed by using the pump of the anti-lock control system, for example, the drop of the braking force cannot be suppressed so readily by sensing the brake operating condition through a master pressure sensor of the system and controlling the master pressure to a target value, because the depressing force and the stroke for generating the master pressure via the brake pedal operation are great. Therefore, the brake becomes inoperable until the master pressure recovers, and a driver may feel loosing of braking force.